add another error message test

Author: zth

tests/build_tests/super_errors/expected/let_unwrap_return_type_mismatch.res.expected

@@ -0,0 +1,15 @@
+
+  We've found a bug for you!
+  /.../fixtures/let_unwrap_return_type_mismatch.res:4:8-14
+
+  2 │ 
+  3 │ let fn = (): int => {
+  4 │   let? Some(x) = None
+  5 │   42
+  6 │ }
+
+  This has type: option<'a>
+  But this switch is expected to return: int
+
+  All branches in a switch must return the same type.
+  To fix this, change your branch to return the expected type.

tests/build_tests/super_errors/fixtures/let_unwrap_return_type_mismatch.res

@@ -0,0 +1,6 @@
+@@config({flags: ["-enable-experimental", "LetUnwrap"]})
+
+let fn = (): int => {
+  let? Some(x) = None
+  42
+}